The objectives of this study were to evaluate the optimum planting density of quinoa (Chenopodium quinoa Willd.) variety NL-6 from the standpoint of efficiency for light utilization, dry matter production and seed yield. Field experiments were carried out in the experiment field at Nihon University in 2012, 2013 and 2014. In 2012, the row width was 50 cm, and the planting density was from 50 to 300 plants m^-2. In 2013, the row width was 30 cm, and the planting density was from 100 to 400 plants m^-2. In 2014, the planting density was 100 plants m^-2, and the row width was from 15 to 60 cm. In the case of 50 cm row width, there were no significant differences in the extinction coefficient and relative illuminance on the soil surface at flowering and seed filling stage, in crop growth rate (CGR) from flowering to seed filling stage, and in seed yield among plots. In the case of 30 cm row width, the extinction coefficient and the relative illuminance on the soil surface in 300 and 400 plants m^-2 were higher than those in 100 and 200 plants m^-2, and the relative illuminance on the soil surface was lower at flowering and seed filling stage. However, there were no significant differences in the CGR from flowering to seed filling stage and seed yield among plots. The extinction coefficient in the 15 and 30 cm row width plots was higher than that in the 60 cm row width plot, and the relative illuminance on the soil surface was lower at flowering and seed filling stage. At a lower planting density and wider row width plots, the seed yields in the branch were higher than that in higher planting density and narrower row width plots. Therefore, we concluded that quinoa variety NL-6 needs from 50 to 100 plants per m^-2 to get high seed yield regardless of planting density and row width. Seed yield was significantly correlated with the top dry weight at the seed filling stage (r=0.701*). Thus, the heavy dry weight at the seed filling stage is very important to increase the seed yield of quinoa variety NL-6.